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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"



titrating the excess with sodium arsenite, prefer the simpler method of direct
titration : the permanganate solution is added little by little to the boiling liquid
and the precipitate allowed to settle after each addition, this being continued
until the supernatant liquid exhibits a persistent pink colour. In this case, also,
a preliminary trial titration is, of course, necessary. It appears, however, that
under these 'conditions, intermediate oxides of manganese may be formed, so
that the results are not always exact.

(6) ELECTROLYTIC DETERMINATION.11'5 gram of the finely powdered
alloy is treated, in a covered porcelain beaker of about 100 c.c. capacity,
with 30 c.c. of nitric acid (D 1-2) containing a few drops of hydrochloric
acid. At the end of the action, the clock-glass and the edges of the beaker
are washed with water, 1-2 grams of ammonium nitrate added and the
liquid evaporated on a water-bath to a syrup and then carefully over a
small direct flame to redness.

When cold, the oxides of iron and manganese are dissolved in 4-5 c.c.
of cone, hydrochloric acid in the hot, 10 c.c. of 50% sulphuric acid being
added to the cooled liquid and the solution heated on a sand-bath until
the hydrochloric acid is completely expelled and copious white fumes appear.
After being heated with water to dissolve the sulphates of iron and man-
ganese, the liquid is filtered and the filtrate collected in a 250 c.c. measuring
flask and the beaker and filter washed repeatedly with boiling water acidified
with sulphuric acid.

The filter then contains the silica, contaminated by small quantities of
graphitic carbon. If the silicon content is required, the procedure given
on p. 171 is followed.

The liquid in the flask is made up to 250 c.c. and 50 c.c. (=0-3 gram
of the alloy) z treated in a 100 c.c. beaker with ammonia until the iron
begins to precipitate. The liquid is then heated on a steam-bath and
dilute sulphuric acid added drop by drop until the ferric hydroxide is com-
pletely dissolved. The solution of ferric and manganese sulphates is then
poured into a solution of 6-7 grams of ammonium oxalate in a little boiling
water contained in an electrolytic cell (not too narrow), 5-6 c.c. of 2%
hydrazine sulphate solution being added and the liquid diluted to 200 c.c.
and subjected to electrolysis to deposit the iron : ND100 == 0-7 amp., voltage
= 4-4-5, duration = 3-5 hours, Winkler electrodes (see later : Electrolytic
analysis of metals).

When the electrolysis has commenced, 2% hydrazine sulphate solution
is allowed to drop in the neighbourhood of the anode from a small tap-
funnel drawn out to a capillary (8-10 drops per minute), this addition
being continued uninterruptedly throughout the electrolysis.

As soon as the liquid loses its yellow tint and becomes completely colour-
less, a drop is removed, treated with a drop of dilute nitric acid, 2-3 c.c.
of hydrochloric acid and as much ammonium thiocyanate solution. When
only a barely perceptible pink coloration is thus obtained, the deposition
of the iron may be regarded as complete.

1  Belasio :   " Separation of Iron from Manganese Electrolytically,"    Ann. Labor.
Chim. Gabelle, 1912, VI, p. 207.

2  With ferro-manganese containing much manganese, 25 c.c.,   corresponding with
0-15 gram of the alloy, are taken.large excess, -which,